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Composition is when one object is made up of another object and the other object could not exist without the first.
A person is made up of body parts such as an arm or foot. An arm, foot etc can't exist without a person. If the person died, so too would the body parts.
Aggregation is when an object is made up of other objects but those objects can still exist on their own.
A classroom is made up of students and a teacher. If a student left the classroom the classroom would still exist as would the student.